Accurate "Thermometers" in Space

Accurate "Thermometers" in Space

The State of Climate Measurement Science

October 2, 1997

Just how accurate are space-based measurements of
the temperature of the Earth's atmosphere? In a recent edition of Nature,
scientists Dr. John Christy of
the University of Alabama in Huntsville, and Dr.
Roy Spencer of NASA/Marshall describe in detail just how reliable these
measurements are.

Why is it important?

The question is very important, as these temperature
measurements from satellites in space are one of our most important windows
into measuring and understanding the phenomenon of Global Warming.

Over
the past century, global measurements of the temperature at the Earth's
surface have indicated a warming trend of between 0.3 and 0.6 degrees C.
But many - especially the early -
computer-based global climate models (GCM's) predict that the rate should
be even higher if it is due to the man-made "Greenhouse Effect".
Furthermore, these computer models also predict that the Earth's lower atmosphere
should behave in lock-step with the surface, but with temperature increases
that are even more pronounced. (Get
the latest on the Earth's Temperature from Space by clicking on the diagram!!)

What is the "Controversy"?

Unlike the surface-based temperatures, global temperature measurements of the Earth's
lower atmosphere obtained from satellites reveal no definitive warming
trend over the past two decades. The slight trend that is in the data actually
appears to be downward. The largest fluctuations in the satellite temperature
data are not from any man-made activity, but from natural phenomena such
as large volcanic eruptions from Mt. Pinatubo, and from El Niño.
So the programs which model global warming in a computer say the
temperature of the Earth's lower atmosphere should be going up markedly,
but actual measurements of the temperature of the lower atmosphere
reveal no such pronounced activity.

How do we know
the Satellite Data are Correct?

In theory, one
could argue that the computer models are accurate, and that the real measurements
have some problem. However this is not the case. An incredible amount of
work has been done to make sure that the satellite data are the best quality
possible. Recent claims to the contrary by Hurrell
and Trenberth have been shown to be false for a number of reasons, and
are laid to rest in the September 25th edition of Nature
(page 342). The temperature measurements from space are verified by two
direct and independent methods. The first involves actual in-situ
measurements of the lower atmosphere made by balloon-borne observations
around the world. The second uses intercalibration and comparison among
identical experiments on different orbiting platforms. The result is that
the satellite temperature measurements are accurate to within three one-hundredths
of a degree Centigrade (0.03 C) when compared to ground-launched balloons
taking measurements of the same region of the atmosphere at the same time.

The atmosphere is extremely
complex in its behavior. Because of this, finding the correct explanation
for the behavior we observe is complex as well. Virtually all scientists
will agree that a doubling of the amount of carbon dioxide in the Earth's
atmosphere should have some effect on the temperature of the Earth.
But it is much less certain how or if we will recognize the effects
of this increase. There are several reasons:

First, the influence of a man-made doubling of the amount of carbon
dioxide in the atmosphere is small compared to the Earth's natural cooling
rate, on the order of only a percent.

Second, there is a much more important greenhouse gas than carbon dioxide,
namely water
vapor. Water vapor over the Earth is extremely variable, both in space
and in time.

Third, the ways in which clouds and water vapor feed back and ultimately
influence the temperature of the Earth are, at best, poorly understood.

Fourth, while the whole Earth is indeed in a state that scientists
describe as "radiative equilibrium," where the incoming sunlight
equals the outgoing infrared radiation to provide a roughly constant overall
temperature, the surface is far from this radiative balance condition.
Evaporation and convection processes in the atmosphere transport heat from
the surface to the upper troposphere, where it can be much more efficiently
radiated into space since it is above most of the greenhouse-trapping water
vapor. So in short, it is this convective overturning of the atmosphere
- poorly represented in computer models of global warming - that primarily
determines the temperature distribution of the surface and upper troposphere,
not radiation balance.

The Answer Lies
Partly in a Better Understanding of Water's Role

A computer model is only as reliable as the physics
that are built into the program. The physics that are currently in these
computer programs are still insufficient to have much confidence in the
predicted magnitude of global warming, because we currently don't understand
the detailed physical processes of clouds that will determine the extent
and nature of water vapor's feedback into the Earth's temperature.

And the Intergovernmental Panel on Climate Change (IPCC) agrees:

``Feedback from the redistribution of water vapour
remains a substantial uncertainty in climate models...Much of the current
debate has been addressing feedback from the tropical upper troposphere,
where the feedback appears likely to be positive. However, this is not yet
convincingly established; much further evaluation of climate models with
regard to observed processes is needed."

Improving our understanding
of the potential magnitude and extent of any man-made global warming will
require a significant amount of critical scientific investigation, both
in space and on Earth, using both observational and computational analysis
techniques. It is clear that if we've learned anything in the past two decades,
it's that the response and dynamics of the Earth as a complex, interconnected
machine are far more detailed, intricate, and complicated than we first
envisioned. Through NASA's Earth Observing System, researchers will continue
to improve our ability to monitor the Earth system so that we may understand
the subtleties of variations in the global atmosphere. NASA's continued
direct observations of the Earth will help enable us to sort out the complicated
issues of climate variability and change that affect the planet.